1. Field of the Invention
The present invention relates to a wavelength router for setting a route of a wavelength-multiplexed light signal for each wavelength.
2. Description of Related Art
Of wavelength routers, various types have been known, including an array waveguide diffraction grating element, and an element using grating.
In document 1, xe2x80x9cOptronics (2000), No. 2, pp. 139-143xe2x80x9d, an example of an array waveguide diffraction grating element is mentioned. Array waveguide diffraction grating elements have been the subject of vigorous research, since a plurality of input ports can be set and a single level operation is possible.
The array waveguide diffraction grating element has a configuration where two star coupler-shaped flat waveguides are connected with channel waveguides (connecting waveguides), which lengths are different from each other. The input/output ports are connected to ports on the side of the flat waveguide opposite to the connecting waveguides.
The wavelength dispersion characteristic is an index to measure the performance of an element. This indicates how much the direction of the optical beam changes with respect to the change of wavelength. The deflection direction of a beam is determined by the relationship of the optical path length difference between adjacent connecting waveguides and the width between the guides. As the optical path length difference increases and as the width d between the waveguides decreases, the deflection angle d "THgr"r increases. The dispersion is given by the following formula (a).
xcexd"THgr"r/dxcex=xe2x88x92xcex94L/d . . . (a)
Here xcex94L is an optical path length difference between the adjacent connecting waveguides. In the case of array waveguide diffraction grating, the cross-talk characteristic aggravates if optical coupling is generated between the connecting waveguides, so a 20 xcexcm or more of space d is required between the connecting waveguides. Formula (b) shows the relationship of xcex94L with the filter transmission area width xcex94F.
xcex94L=2xcex2/(Nxcex94xcexF)xe2x80x83xe2x80x83(b)
Here N is the number of connecting waveguides. Therefore the following formula (c) is established.
d"THgr"r/dxcex=xe2x88x922xcex/(Ndxcex94xcexF)xe2x80x83xe2x80x83(c)
To have a good cross-talk characteristic, N must be 100 or more, so dispersion cannot be increased very much. But according to formula (c), dispersion can be increased by decreasing d.
For reference, the length L of the star coupler is calculated by the following formula (d).
L=(xcex94xcexF/xcex94xcexC)(NDd)/(2xcex)xe2x80x83xe2x80x83(d)
Here D is a space of the output ports and xcex94xcexc is a wavelength difference between the wavelength channels. As the width d of the waveguides increases, the length L of the star coupler increases. For example, if xcex94xcexF/xcex94xcexc=1, d/xcex=20, D=30 xcexcm and N=100, then L=30 mm.
With the foregoing in view, it is an object of the present invention to provide a compact element with a high wavelength resolution by increasing dispersion by minimizing the space d between the connecting waveguides.
A wavelength router, according to the present invention, comprises two first waveguides, which have a plurality of input/output sections and a plurality of connecting sections, having a structure such that light entering from each of the input/output sections diffuses toward each of the connecting sections, a plurality of second waveguides, and optical path converters which guide light from the first waveguides to the second waveguide, or guide light from the second waveguides to the first waveguides, wherein each of the connecting sections of the two first waveguides are inter-connected via the plurality of second waveguides, and the distances from the plurality of light input/output sections to each of the connecting sections are different from each other, and the optical path converters are disposed at the connecting sections.
According to this configuration, disposing the optical path converters decreases the width between the second waveguides compared with a conventional configuration. As a result, the wavelength dispersion characteristic improves. Therefore the length of the first waveguides can be decreased.